1. Field of the Invention
The present invention relates to a grid electrode at which an opening pattern is formed, a scorotron charger, and an image forming device.
2. Description of the Related Art
A device utilizing corona discharge is generally used as a charger of an image forming device. Corotron chargers and scorotron chargers are known as corona discharge devices. In a corotron charger, a wire electrode spans the interior of a shield case, substantially parallel to a photosensitive body. High voltage is applied to the wire electrode such that corona discharge is caused, and charges are applied to the surface of the photosensitive body. A corotron charger has the drawback that it is easy for fluctuations to arise in the potential of the surface of the photosensitive body due to the accuracy of the charger, errors in setting, and the like. Scorotron chargers, which are advantageous in that they can charge to a uniform potential, have been used recently.
The main portion of a scorotron charger is structured by a discharge electrode which supplies charges to a photosensitive body and charges the photosensitive body, a grid electrode disposed between the discharge electrode and the photosensitive body and controlling the potential of the photosensitive body, and a shield case which is supported so as to cover the discharge electrode. An opening pattern is formed at the grid electrode in order to carry out potential control well. The charge potential of the surface of the photosensitive body can be controlled so as to be uniform by applying a high voltage to the discharge electrode and simultaneously applying the appropriate voltage (i.e., the voltage desired to be charged to) to the grid electrode (see, for example, Japanese Patent Applications Laid-Open (JP-A) Nos. 2001-13765 and 8-36289). Note that, in order to prevent white deletion or the like (there are also cases in which black stripes are generated) from arising in the image due to discharge products such as O3, NOX, and the like which are generated at the time of discharging, there are cases in which the grid electrode is subjected to a surface treatment such as painting or the like in order to decompose the discharge products.
In order to control the potential of the photosensitive body well, it is preferable to make the opening ratio along a straight line, which runs along the moving direction of the photosensitive body (hereinafter called “opening ratio on the line of movement of the photosensitive body”), uniform at all of the regions at which the opening pattern is formed. Here, “opening ratio on the line of movement of the photosensitive body” means the ratio of the distance which a given portion of the photosensitive body moves over while opposing the opening portions, among the distance that that given portion of the photosensitive body moves over while opposing the grid electrode. If the opening ratio on the line of movement of the photosensitive body differs greatly in accordance with the position of the aforementioned portion of the photosensitive body, it is easy for the discharge distribution to become non-uniform, which is not preferable. Such a phenomenon is particularly marked in cases in which the discharge electrode has deteriorated or the discharge electrode and the grid electrode have become dirty. Further, there is also the drawback that the costs required for maintenance of the charger are high due to deterioration of the discharge distribution. Note that, in cases in which there is dispersion in the opening ratio on the line of movement of the photosensitive body, generally, the average value of the opening ratio on the line of movement of the photosensitive body is defined as the opening ratio on the line of movement of the photosensitive body at the entire grid electrode.
As an example of aiming to make uniform the opening ratio on the line of movement of the photosensitive body, for example, JP-A No. 2001-13765 discloses a grid electrode 78 (see FIG. 9) in which this opening ratio is uniform and the ability to control the charge potential is excellent. Long, narrow openings 80, at which the positions of the end portions thereof are uniform, are formed in a fixed direction in the grid electrode 78.
However, in this grid electrode 78, because the long side connecting portions, which connect the long sides of the openings, are extremely long, there is the problem that it is easy for these long side connecting portions to deform. In particular, in a scorotron charger having a mechanism for cleaning the grid electrode 78, this problem arises markedly because, generally, the grid electrode is cleaned by a cleaning member being moved in a direction orthogonal to a moving direction X of the photosensitive body, in a state in which the cleaning member abuts the grid electrode 78. Further, when an opening corner portion 82 which is acute-angled is formed at an opening edge 81 forming the opening 80, there is the problem that the cleaning member (which is mainly bristles) is severed by the opening edge portion forming this acute-angled opening corner portion 82, and image defects arise. There is also the problem that the grid opening ratio cannot be made that large, and the controlling ability of the grid electrode 78 is insufficient.
In order to improve the mechanical strength of a grid electrode, it has been thought to use a grid electrode 88 such as shown in FIG. 10 and disclosed in Japanese Utility Model Application Publication (JP-Y) No. 3-042443. This grid electrode 88 is formed by a screen-like thin plate material having a larger number of minute openings 90 which are shaped as regular hexagons. Among the six sides of the openings 90, two sides are disposed so as to be oriented parallel to the moving direction X of the photosensitive body. In this way, because the configurations of the openings 90 of the grid electrode 88 are regular hexagons, small openings can be arranged densely.
However, the grid electrode 88 disclosed in JP-Y No. 3-042443 has the following problems. Two sides of the regular hexagonal opening 90 of the grid electrode 88 are disposed so as to be oriented parallel to the moving direction X of the photosensitive body, and strip-shaped portions 94, which are electrode portions between adjacent openings along the moving direction X of the photosensitive body, are lined up in one row at uniform intervals. On the other hand, the regions of the surface of the photosensitive body, which regions do not oppose the strip-shaped portions 94 (e.g., the regions of the photosensitive body surface opposing the regions 96 in FIG. 10A), are inclined at 30° or 150° with respect to the direction orthogonal to the rotating direction of the photosensitive body. Accordingly, with such an arrangement and configuration of openings 104, the dispersion in the opening ratio on the line of movement of the photosensitive body is large, and the controlling ability of the grid electrodes is poor (see FIGS. 10B and 10C).
In order to make the dispersion in the opening ratio on the line of movement of the photosensitive body smaller, there are methods such as making the widths of the strip-shaped portions 94 narrower so as to change the configurations and sizes of the openings 90, and the like. However, if the widths of the strip-shaped portions 94 are made narrow, the strength of the grid electrode deteriorates, and other problems arise, such as a predetermined controlling ability cannot be maintained when the openings 90 are made large, and the like.
Further, when the opening ratio on the line of movement of the photosensitive body is the same, the smaller the grid pitch, the better the ability to control the charge potential. (The grid pitch is the distance between both ends, in the moving direction X of the photosensitive body, at each opening portion, and is hereinafter abbreviated as “GP”.) However, if the openings 90 are shaped as regular hexagons as described above, a problems arises in that, as compared with openings of the same opening ratio but other configurations, the GP is large, which is not very preferable in terms of the controlling ability (see FIGS. 10B and 10C).
In order to suppress the dispersion in the opening ratio on the line of movement of the photosensitive body, it has been thought to adjust the orientation of the regular hexagon shapes (see, for example, JP-Y No. 62-181954). However, as compared with openings of the same opening ratio but other shapes, an increase in the GP is unavoidable.